This invention relates to an explosion-proof, pollution-free infrared dryer for concentrated vaporization and removal of combustible or flammable solvents from a newly coated product, such as a newly printed web of paper or a newly applied adhesive on a web of material.
Removal of a combustible solvent from a newly applied coating involves at least three problematic considerations. First, known methods which generate necessary temperatures in the range of 180.degree.-450.degree. F. are expensive and inefficient in operation. Secondly, precautions must be exercised because the solvents are combustible and may cause explosions. Finally, release of vaporized solvents into the atmosphere causes pollution.
Drying newly applied coatings dispersed with flammable solvents has been accomplished in the past by hot air impact drying ovens. This method works reasonably well but has its drawbacks. Explosion control is typically accomplished by dilution or scattering of the combustible solvent molecules in vast volumes of heated air to extremely low concentration levels. This method is expensive in that it involves heating up high volumes of air for rapid circulation about the coating to be dried. Thereafter, the vast volumes of solvent-laden hot air must cooled so that the widely scattered solvent molecules can be recaptured or condensed.
Infrared radiant heaters, such as the fuel-fired Schwank-type, are known to be used for drying products and for heating various enclosures. Infrared is unique in that it is not heat but rather radiant energy that generates heat when it enters an absorbent material much like electricity flowing into a resistor. Exemplary of such heaters are those shown in my prior U. S. Pats. Nos. 3,315,656; 3,797,474 and 3,849,063. Gas-fired infrared is desirable because it costs one-third to one-half as much as electric infrared in most instances, which in turn generally costs less than hot air impact ovens that are utilized in drying materials.
Combustible solvents typically used in printing and applying inks, adhesives and the like, are highly absorbent of infrared. They have many carbon-carbon and carbon-hydrogen bonds. These bonds make the solvents highly absorbent of infrared in the 2-4 mu range. Ink pigments are also highly absorbent of infrared. White coated paper, however, absorbs very little infrared.
Hot air impact drying and infrared drying complement each other. Newly printed ink dispersed with a combustible solvent is comprised essentially of various sized dots printed on a web of paper. For very tiny dots of ink hot air impact drying works reasonably well. This is so because the very tiny dots have a great deal of surface per unit weight which will permit losses of its solvent readily when heated by conduction. Large ink dots or masses, on the other hand, have just the opposite quality. Their low surface-mass ratio makes conduction drying difficult. However, under infrared radiation heat readily builds up within a mass undergoing infrared absorption. The heat drives the solvent out of the mass into a vaporized state. Thus, the two drying methods used together would complement each other to remove a combustible solvent from a newly printed web.